Solid state imaging devices such as charge coupled devices (CCDs) and complementary metal oxide semiconductor (CMOS) image sensors are widely used in imaging applications ranging from cameras to mobile telephones and computers. Because CMOS manufacturing technology is compatible with the formation of other semiconductor devices, it is possible to integrate CMOS image sensors with other devices.
CMOS image sensors fall broadly into two categories: front side illumination sensors and back side illumination sensors. In front side illumination devices, the photodiode that captures the photons is positioned relatively far from the incident light. Thus back side illumination devices, in which the photodiode is positioned nearer to the incident light, have increasingly been developed. However, the circuitry for driving the imaging sensor has conventionally been positioned in regions between adjacent photodiodes, limiting the area for light capture available to the photodiode (that is, limiting the “fill factor” of the device, the ratio of the area of the photodiode to the area of the pixel). Laterally positioned circuitry that competes for chip cross-sectional area with the photodiode thus significantly reducing the sensing area.
Further, conventional CMOS image sensors combine the use of smaller photodiodes (lower “fill factor” photodiodes) with microlenses in an attempt to reduce optical crosstalk between adjacent pixels. However, the use of microlenses requires a pixel size of at least on the order of 1.2 microns. Due to this minimum pixel size, increased pixel density with pixels of smaller sizes is impossible, thereby setting a limit on image resolution for image sensors using microlenses.
Electrical cross-talk is also a problem impeding the development of smaller pixel sizes. Smaller pixel sizes results in a higher pixel density per unit area, increasing the problem of both electrical and optical crosstalk between neighboring pixels.
Thus there is a need in the art for improved CMOS image sensors with larger photodiode fill factors that do not require microlenses, thereby permitting the fabrication of images sensors with pixel sizes less than 1.2 microns and increased pixel density. There is also a need in the art for improved CMOS image sensors capable of supporting extremely high pixel density (due to smaller pixel sizes) without high optical and electrical crosstalk.